It may be hard to imagine today, but on 9/11 the thought actually crossed my mind that America’s social divisions would now melt away, or at least radically diminish. After the fall of the Twin Towers, how could anyone continue to believe (or pretend to believe) that gays, for example, were a real threat to America? Surely the U.S. would unite in defense of its freedoms—everybody’s freedoms—and in opposition to the jihadists. For a moment, that seemed to be happening. Then the finger-pointing started. Leftists railed that America had gotten its payback for imperialism; Jerry Falwell insisted that pagans, abortionists,...